The present invention relates to refrigerated display cases having an ambient air defrost system. More particularly, the invention relates to an air defrost system in which adjacent air bands flow in opposite directions in defrost. This causes the bands to collide and direct a part of the air outside the case.
All references herein to refrigeration apparatus or refrigeration operations are intended to include cooling at a temperature below 32.degree. F., such as associated with frozen food display cases, or below 0.degree. F., such as with ice cream cases, or in excess of 32.degree. F., such as typically associated with dairy, fresh meat and produce display cases.
In the operation of commercial refrigerated display cases, of the type found for example, in supermarkets and the like, it is desirable to include a system capable of automatically defrosting the display case. Preferably, the defrost cycle is actuated either at preset periodic times or when the frost buildup within the system has reached a certain predetermined level. The system may be controlled to begin the defrost operation at a preset time or times as set on a master control clock; defrost termination is usually thermostatically controlled, with a fail-safe clock-controlled maximum defrost time period. Alternatively, the system may be thermostatically controlled so as to switch from the refrigeration cycle to the defrost cycle when a preset level of frost buildup is detected. By either manner of operation, it is possible to avoid significant frost buildup within the display case.
Typically, three main approaches have been employed in the past for defrosting refrigerated display cases. The first approach involves the use of electric resistance heaters that are arranged adjacent to the refrigeration coils of the refrigeration mechanism. During a defrost cycle, these heaters are energized to radiate heat in an effort to eliminate the frost buildup on the coils; this also adds heat to the air being circulated through the conduit within the case. This electric defrost is relatively simple both in construction and operation. No additional moving parts are required, although it is generally necessary to install an additional three-phase 220 V. line for the heater circuit. The electrical heaters are high wattage heaters, and thus utilize a substantial amount of electricity during operation. Furthermore, the additional warm air being circulated in the case due to the radiant heat from the heaters can raise the temperature of the case above desirable limits, thereby increasing the risk of product spoilage.
A second type of defrost system in use circulates relatively high temperature, high pressure compressed gaseous refrigerant through selected evaporator coils in the defrost cycle in the opposite direction to the flow of refrigerant through the evaporator coils during the refrigeration cycle. During the defrost cycle, a valve mechanism shuts off the supply of low temperature liquid refrigerant to the evaporator coils to be defrosted and instead routes the compressed hot gaseous refrigerant through the coils for defrosting. Gas defrost requires additional expensive mechanical components, including an extra several hundred feet or more of refrigerant carrying conduit, valves, solenoids, etc. All of these elements are subject to rapid and extreme temperature shifts, and resulting expansion, particularly at the start and end of a defrost cycle. Due to the requirement that the system be able to be selectively switchable to supply hot gaseous refrigerant to selected ones (but not all) of the evaporator coils, a complicated valving and control structure must be provided.
A third, relatively recently developed approach to defrosting display cases relies upon naturally warm ambient air. An example of an ambient air defrosting system which has proven to be commercially successful is shown in U.S. Pat. No. 4,144,720 to Subera et al, which is assigned to the same assignee as the present invention. The Subera '720 patent discloses an open front refrigerated display case having primary and secondary air conduits. During a defrost cycle, the direction of air flow through the secondary conduit is reversed to draw in air from outside the display case. This ambient air is directed into the primary band conduit where it is forced to flow over the evaporator coils and defrost them. A feature of the system shown in the Subera '720 patent is that the primary band air flow is continuously maintained in both the refrigeration and defrost cycles, whereby an air curtain is maintained across the access opening at all significant times.
Other reversible fan air defrost systems are shown in U.S. Pat. No. 4,026,121 to Aokage et al and U.S. Pat. No. 4,120,174 to Johnston. Other air defrost systems generally are shown in U.S. Pat. Nos. 3,082,612; 3,403,525; 3,850,003; and 3,937,033, all to Beckwith et al.